FIELD : medicine. SUBSTANCE : proposed group of inventions relates to the field of medicine, in particular obstetrics and medical genetics. Methods have been proposed for determining the source of aneuploid cells by the blood of a pregnant woman. By analyzing the sequencing data, a fraction of free circulating (extracellular) DNA fragments is selected that are shorter than 150–200 bp. For the fraction obtained, the proportion of fruit DNA is determined (ffX). In the case of an increase in the proportion of fetal DNA with a decrease in the length of fragments of extracellular DNA selected for analysis, the aneuploidy origin is fetal. In the case of the absence of an increase in the proportion of fetal DNA, the presence of aneuploidy in the mother’s cells. EFFECT : proposed group of inventions provides increased reliability of laboratory research by identifying cases of aneuploidy of the mother itself, including mosaic forms, that are the cause of false-positive results of DNA screening. 2 cl, 1 dwg, 2 ex